A typical wireless communication system includes one or more base stations, each providing respective wireless coverage in which to serves user equipment devices (UEs) such as cell phones, computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices (whether or not operated by a human user). In turn, each base station could be coupled with network infrastructure, including one or more routers, gateways, and/or switches, that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or a packet-switched network such as the Internet. With this arrangement, a UE within coverage of the system could engage in air interface communication with a base station and could thereby communicate via the base station with various remote network entities or with other served UEs.
In practice, such a system could be configured to operate in accordance with a radio access technology, examples of which include without limitation Long Term Evolution (LTE) (using Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), WIFI, BLUETOOTH, and others. Each technology could define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and/or other operations.
Under a representative radio access technology, the air interface between the base station and served UEs defines physical air interface resources that can carry data between the base station the UEs. For example, the air interface could define one or more frequency channels, carriers, or subcarriers on which data can be modulated for transmission. Further, the air interface could be divided over time into intervals in which transmission can occur, and the air interface could define particular time-frequency resources, each occupying particular frequency bandwidth and spanning a particular time interval. The representative radio access technology could define such air interface resources on a downlink for carrying data from the base station to UEs and on an uplink for carrying data from UEs to the base station.
In operation, the base station could then be configured to coordinate use of these air interface resources on an as-needed basis. For example, when the base station has data to transmit to a particular UE, the base station could allocate particular downlink air interface resources to carry that data and could accordingly transmit the data to the UE on the allocated downlink resources. And when a UE has data to transmit to the base station, the UE could request an uplink resource grant, the base station could responsively allocate particular uplink air interface resources to carry the data, and the UE could then transmit the data to the base station on the allocated uplink resources.
One of the key performance metrics of a wireless communication system is its spectral efficiency, namely, the extent of data that the system can carry per frequency spectrum. For instance, the spectral efficiency of a wireless communication system could be measured as a quantity of bits per Hertz.
To help improve spectral efficiency, a base station could be configured to implement Multi-User Multiple-Input-Multiple-Output (MU-MIMO) for its air interface. With MU-MIMO, the base station uses spatially diverse radio-frequency transmission paths to communicate with multiple UEs (typically a pair of UEs) on overlapping/shared air interface resources. For instance, the base station could allocate the same time-frequency resources to two UEs, so that the base station would communicate with the two UEs on the same frequency and at the same time. To facilitate this, the base station could select a pair of UEs that are sufficiently orthogonal to each other, such as UEs that each report a sufficiently high signal-to-noise-plus-interference ratio (SINK), to help avoid interference between their respective communication paths. The base station could then schedule communications with both UEs to occur on the same time-frequency resources, using appropriate precoding and spatial diversity or other mechanisms to help distinguish the communications from one another.